1. Field of the Invention
The present invention relates to an image forming apparatus having a plurality of developing devices and, more particularly, to an image forming apparatus having improved means for controlling the amount (which will be called the "toner application amount") of toner applied to a latent image on an image retainer from each of the developing devices.
2. Description of the Prior Art
In the image forming apparatus having a plurality of developing devices according to the prior art, the toner application amount is controlled by changing the developing bias voltage of each developing device. Specifically, the developing bias voltage is raised, in case the toner application amount is to be increased, and is dropped in case the toner application amount is to be decreased.
A color image has its color reproduction determined depending upon the toner application amount on a photosensitive member and the developing order. Especially in case a superposition is to be accomplished on the photosensitive member by the reversing development, it is necessary to meet all the various restrictions upon the development such as the image fog, resolution and density. Especially in the two-component development, the condition of carrier application has also to be met.
In case a full-color reproduction is to be accomplished by superposing the toners, moreover, the control of the developing bias for satisfying those restrictions is especially difficult.
The toner application amount control means of the conventional image forming apparatus thus far described is troubled by the following problems. Specifically, if the developing bias voltage exceeds a particular level with a view to increasing the toner application amount, the carrier is caught by the formed image to rough the image surface, or excess toner is applied to the image (to fog the image with the toner) to dirty that portion of the image, which should be intrinsically white or to drop the resolution. Another problem in the full-color image case is to narrow the color reproduction range.
FIG. 4 plots the amount of toner applied to a second layer against that to a first layer. The fourth quadrant plots the potential change against the toner application amount on the first layer when the toner (i.e., the toner on the first layer), which has been subjected to the charging .fwdarw. exposure .fwdarw. development so that it is applied to the photosensitive member, is further subjected to the recharging .fwdarw. reexposure. The potential drop in FIG. 4 indicates the difference between the charging potential and the surface potential at the reexposed portion. The third quadrant plots the potential drop against the developing potential (i.e., the difference between the DC voltage of the developing bias and the surface potential at the reexposed portion). The second quadrant plots the toner application amount on the second layer against the developing potential. This plotted relation presents a characteristic curve for determining the toner application amount on the second layer in case the two toners are superposed on the photosensitive member.
In the case of the full color image, the toner application amount is an important parameter for determining the color reproducing range.
In order to control the toner application amount, it is sufficient to change the characteristic curve of the second quadrant. However, the potential at the reexposed portion, i.e., the parameter for determining the developing potential is determined by the amount of charge of the toner or the like. On the other hand, the DC voltage of the developing bias is also under a substantially determined condition for preventing the toner fog, carrier catch and resolution drop. In order to control the toner application amount under this circumstance, another parameter has to be sought for.
What is required for controlling the toner application amount is the color reproducing range and the monochromatic image density, as shown in FIG. 3. It is therefore preferable to use the image density over a predetermined level. This should take into consideration in the control of the attached toner amount.